1. Field of the Invention
The present invention, relates to a high-side driver circuit, more particularly to a high-side driver circuit combined with the bootstrap circuit and the depletion-mode transistor.
2. Description of the Prior Art
In the common high-efficiency and high-power system, the N-type device with better performance is used as the high-side power transistor. Thus, the source of the power transistor is a floating voltage, which is unable to be used to control the switch of the transistor directly via a constant gate voltage. The conventional method for solving the problem is that to adopt a bootstrap circuit to float the gate voltage with respect to the source voltage. The design for the high-side power transistor with the conventional bootstrap circuit is only applicable for the enhancement-mode transistor and is not the depletion-mode transistor, because the charging is not enough for the bootstrap capacitor upon driving the depletion-mode transistor.
Please referring to FIG. 1 which is a diagram illustrating the conventional high side gate driver with the bootstrap circuit. The bootstrap capacitor containing high side gate driver 100 is used to drive the enhancement-mode transistor Mtop. The operation principle is described as the followings: When the enhancement-mode transistor Mtop is turned off, the input voltage VCC will charge the first capacitor C1. In the first capacitor C1, the voltage of negative end (i.e. the source voltage of the enhancement-mode transistor Mtop) is zero, and the voltage of positive end is VCC. When the first transistor M1 is conducted, the gate-source voltage VGS of the enhancement-mode transistor Mtop is VCC. Thus, the enhancement-mode transistor Mtop is conducted, and the source voltage is increased to VDD. The gate-source voltage VGS of the enhancement-mode transistor Mtop is maintained at VCC via C1, in order to conduct the enhancement-mode transistor Mtop continuously. When the first transistor M1 is turned off, and the second transistor M2 is conducted, the gate-source voltage VGS of the enhancement-mode transistor Mtop is zero. Thus, the enhancement-mode transistor Mtop is turned off.
The design for this circuit is only applicable for the enhancement-mode transistor. When the power transistor is the depletion-mode transistor, the transistor is conducted because the gate-source voltage VGS of the depletion-mode transistor is zero. Thus, a circuit design applicable for the depletion-mode transistor is provided for the high side gate driver with the bootstrap circuit.
In recent years, many high performance power devices developed prosperously are the depletion-mode devices. One driver design of conventional depletion-mode transistor combines the MOSFET and high performance depletion-mode device to become the enhancement-mode device. Then, the driving technique of the existing enhancement-mode transistor is adopted to control the switch of device. However, this design will reduce the performance of the depletion-mode transistor, such as increase the on-resistance of composite device.
The second driver design of conventional depletion-mode transistor uses a floating voltage source to obtain the required negative voltage to control the switch of the depletion-mode transistor. However, this design requires a floating voltage source, which is more complicated.
The third driver design of conventional depletion-mode transistor uses the switch of a capacitor and relevant transistors to produce a negative voltage to control the switch of the depletion-mode transistor. However, this design is not applicable for the high-side driver circuit with the bootstrap circuit.
Therefore, the present invention provides a high side gate driver with the depletion-mode transistor. This circuit can combine the bootstrap circuit to complete the switch of the depletion-mode transistor without reducing the performance of the depletion-mode transistor and the need of a floating voltage source or a negative voltage source.